1. Field of the Invention
The present invention relates to a semiconductor module having an electronic component placed on a module board, and in particular, to a semiconductor module having a top panel which is disposed to face a module board with an electronic component therebetween to be adhered to the electronic component with adhesive.
2. Description of Related Art
Semiconductor modules having a top panel disposed to face a module board with an electronic component therebetween have conventionally been proposed. For example, JP-U-H07-42126 discloses such a top panel, which is formed with an insulating panel made of a material such as a resin or ceramics (see, for example, paragraph [0010] and FIG. 1 of the above-mentioned publication).
Conventional examples of semiconductor modules having a top panel include one structured such that a legged top panel made of a nickel silver sheet (metal) is soldered to a module board. The top panel needs to be legless to realize a compact module board; however, in a case where a legless top panel is fitted to an electronic component placed on a module board, if the top panel should come off, the top panel (which is made of metal) might come in contact with a component on a main board on which the semiconductor module is mounted, to cause an electrical short circuit.
The above inconvenience caused when the top panel comes off could be prevented by giving the top panel an insulation structure (a structure in which metal is covered with resin). On the other hand, with such a structure, in which the top panel is formed of different materials laid one on top of another, warp occurs in the top panel under temperature change when the semiconductor module is mounted on the main board. This results in mounting failure of the electronic component. This problem will be described below in more detail.
FIG. 5A is a sectional view showing an example of the structure of a semiconductor module having a top panel structured such that metal is insulated with resin. In this semiconductor module, electronic components 103 are placed on a module board 101 with conductor patterns 102 therebetween. The electronic components 103 are adhered (soldered) to the conductor patterns 102 by using solder 104 as necessary. A top panel 105 is disposed to face the module 101 so as to cover the electronic components 103.
The top panel 105 is structured such that a metal layer 105b is adhered to one side of a resin layer 105a, on which a solder resist 105c is further provided so as to coat the metal layer 105b. This structure helps secure stiffness and an insulating characteristic of the top panel 105.
On the other hand, FIG. 6A is a sectional view showing another example of the structure of the semiconductor module having a top panel, structured such that metal is insulated with resin. In this semiconductor module, the metal layer 105b is formed at a side of the resin layer 105a opposite from the side on which the metal layer 105b is formed in the top panel 105 shown in FIG. 5A. That is, the top panel 105 shown in FIG. 6A is structured such that the metal layer 105b is adhered to the resin layer 105a at a side thereof which is closer to the electronic components, and the solder resist 105c is provided as a coating to cover the metal layer 105b. 
In either case, the top panel 105 is fixed with adhesive 106 to one of the electronic components 103 that is tall and has a surface large enough to secure an adhesion area. If no electronic component 103 is available in the center portion of the module to be adhered to the top panel 105, the top panel 105 is maintained parallel to the module board 101 by being adhered to a plurality of electronic components 103 placed in the peripheral portion of the module.
However, since the resin layer 105a and the metal layer 105b have different coefficients of thermal expansion, temperature change causes warp in the panel 105 structured such that the metal layer 105b is adhered to just one side of the resin layer 105a as shown in FIGS. 5A and 6A. This results in mounting failure of the electronic components 103 in the module when the semiconductor module is adhered to the main board by reflow soldering.
That is, each of the semiconductor modules shown in FIGS. 5A and 6A is mounted on the unillustrated main board by reflow soldering, and the solder 104 and the adhesive 106 with which the top panel 105 is fixed in the module are melted or softened by heat applied to the semiconductor module on the main board during the reflow soldering. In the structure shown in FIG. 5A, if the top panel 105 warps in a state in which the solder 104 and the adhesive 106 are melted or softened, the top panel 105 presses the electronic components 103 down as shown in FIG. 5B. When cooled, the solder 104 is hardened first, and thus the electronic components 103 are fixed in a tilted state.
On the other hand, in the structure shown in FIG. 6A, if the top panel 105 warps in a state in which the solder 104 and the adhesive 106 are melted or softened, the top panel 105, as shown in FIG. 6B, pulls up the electronic components 103 which are fixed thereto with the adhesive 106. And, when cooled, as in the above described case, the electronic components 105 are fixed in a tilted state.